It is the general aim of the proposed research to determine the experimental conditions which will maximize resolution for specific chromosomal site localization in the human (and other mammals) by hybridization in situ. The use of this technique will be extended to include not only the localization of transcriptionally active chromosomal DNA, but also the determination of the general morphological and functional organization of DNA sequences in the chromosomal unit. Of particular interest is the localization of human genes which code for individual species of messenger RNA and transfer RNA. As a corollary, several studies are planned or are in progress: (a) the chromosomal location of viral-specific DNA which is co-valently linked to cellular DNA; (b) the production of mice with predictable ribosomal DNA deficiencies as a means of determining the phenotypic characteristics resulting from such deficiencies in the mammal; (c) a determination of the relationship between rDNA-containing micronucleoli in human diplotene nuclei and an amplification-like phenomenon; (d) the use of hybridization in situ for relative gene quantitation; (e) the delineation of polymorphisms for rDNA and 5S DNA in the human and related primates; (f) a determination of the feasibility of obtaining probes specific for a given stage of development, or probes derived from fractionation and/or transcription in vitro of chromatin-derived nucleic acids; and (g) devising more appropriate and accurate methods of analysis of the results of experiments employing hybridization in situ using computer assistance.